1. Field of the Invention
The present invention relates to a sample inspection apparatus and a sample inspection method for inspecting samples such as a photomask, a wafer, a reticle and a liquid crystal substrate, on which patterns relating to fabrication of semiconductor devices are formed.
2. Description of the Related Art
A main cause in a decrease of the yield of large scale integration (LSI) circuits is a defect in a photomask used in fabricating semiconductor devices by means of lithography. With recent development of LSI, a pattern formed on a photomask has become finer. Accordingly, the dimensions of a defect to be detected are very small. Under the circumstances, apparatuses for inspecting such defects, i.e. sample inspection apparatuses such as a defect inspection apparatus and a pattern inspection apparatus have been widely developed and put into practical use.
A demand for a stricter inspection has increased. For example, in order to exactly detect a very small defect by a defect inspection apparatus, the relationship between a signal level obtained at the time of observing a defect and a noise level in a detection system is very important. In other words, a gain of an optical apparatus or a signal acquiring means must be sufficiently high over a spatial frequency range which is wide enough to detect the defect.
The modulation transmission characteristics of the optical apparatus is determined by a numerical aperture (NA) for determining the amount of optical information needed to form an image, the average wavelength of used light, and the degree of cohesion. If the NA is increased, the spatial frequency band permitting optical resolution increases but the focal depth decreases accordingly. In addition, if an area for observation is unchanged, the size in structure increases. It thus appears that the limit of the NA is, in fact, about 0.8.
To decrease the optical wavelength is effective means for detecting a very small defect. However, since the sensitivity of a detection sensor decreases in a short wavelength region, a custom-made expensive sensor is required and there are many practical problems. In a method of increasing the cohesion of wavelength of light to be used, the MTF relating to intermediate frequencies can be improved but the level of a very weak signal is further decreased.
Besides, there has recently been proposed a method of improving a signal which is degraded due to various factors in an optical defect inspection apparatus. In this method, a measured image signal is subjected to proper filtering (Jap. Pat. Appln. KOKAI Publication No. 61-82107). This method is very effective when an S/N of a certain level is maintained. However, the method of improving the degraded signal emphasizes noise consequently. If this method is actually adopted, an inspection apparatus would output many pseudo-defects (defect-free portions are erroneously determined to be defective portions). Even if this method is adopted, it is necessary to decrease unnecessary noise by some means.
On the other hand, the conventional mask defect inspection apparatus generally adopts two defect inspection methods: 1) two chips with the same pattern are observed by different detection means, and a difference therebetween is detected by comparison by a proper defect detection algorithm, and 2) a chip with a pattern is observed by detection means and the observation result is compared with pattern design data by a proper defect detection algorithm, thereby finding a defect.
In the case of the former, since the two chips with the same pattern are individually observed, the same defect, if being present, cannot be detected.
In the case of the latter, the observation result is compared with the design data, such a problem does not occur. Examples of the inspection apparatus using design data are disclosed in a prior art document (VSLI High Precision Total Automatic Reticle Inspection Apparatus, Electronic Material, September 1983, p. 47) or Jap. KOKU Patent No. 1-40489.
It is desirable that design data used at the time of producing (writing) a reticle coincides with design data to be input to the inspection apparatus to execute an inspection. A highly efficient system can be constructed if these two design data are used in a writing apparatus or an inspection apparatus which is designed and manufactured under the condition that these two design data coincide.
In the meantime, most of the conventional masks to be inspected are formed such that an un-transparent chrome is deposited on a transparent glass, a pattern is written, and the resultant is subjected to etching. Such masks have ideal optical transmissivity of about 100% and about 0%.
In the meantime, in order to improve resolution characteristics of an exposure apparatus, various phase-shift masks have been devised and put to practical use. Among the masks, a mask called "half-tone mask" is prospective because of easy pattern design. The half-tone mask uses a semi-transparent film of silicon nitride, etc. in place of a conventional chrome film. As regards this type of mask, too, a defect in a pattern formed of a half-tone film needs to be inspected, like a conventional chrome mask. However, the transmissivity of the semi-transparent film ranges from about 10% to about 70% or more in some cases. Consequently, sufficient optical contrast cannot surely be obtained, the resolution lowers, and precise defect detection cannot be performed.
In a possible application by a mask designer or a user, a conventional chrome film and a semi-transparent half-tone film may be used in combination.
In the conventional mask inspection apparatus (of a data base comparison system), however, no consideration has been given to the combinatorial use of both films. Specifically, in a conventional defect detection algorithm in which a signal level is used for determination, only two states of a light transmission portion (glass) and a light shielding portion (chrome) are treated. For example, when a chrome/half-tone/glass mask, in which a target pattern portion is formed of a half-tone film and an outer frame is formed of a chrome film, is inspected, a half-tone level differs from a chrome level and may be determined to be defective.
Accordingly, the determination method using a signal level cannot be adopted, and a differentiation comparison method for determining a defect on the basis of light/dark differential values of observation data is adopted. However, it is difficult to set a defect determination threshold value and it is not possible to perform precise defect detection.
Furthermore, there are problems of overshoot and undershoot at the edge portion of a glass/half-tone film. In a conventional defect detection algorithm for chrome masks, such overshoot and undershoot portions are determined to be defective since they depart from an inspection standard created from an actual design pattern image.
Since such overshoot and undershoot will occur at normal edge portions, it is not possible to strictly set the threshold of the conventional defect detection algorithm. Thus, there is a concern that a very small defect to be detected may not be detected.
Under the circumstances, development of an inspection apparatus matching a half-tone mask is to be desired.
As has been stated above, the conventional sample inspection apparatus for detecting defects of patterns of a mask, a reticle, etc. is used, with the detection sensitivity enhanced to such a level as to detect no pseudo-defects or signals corresponding to noise. Moreover, various detection algorithms have been developed to enhance the defect detection ratio, but adequate detection sensitivity cannot be attained.
The pattern defects include isolated defects, edge-portion defects, corner-portion defects, etc. With respect to each defect, there are a remaining portion (a black defect) of a pattern and a missing portion (a white defect) of a pattern. In general, the conventional apparatus has different maximum defect detection dimensions for so many defects. For example, the maximum detection sensitivity of a black isolated defect is 0.3 .mu.m, while the maximum detection sensitivity of a white isolated defect is 0.4 .mu.m. The reason for this is that signal amplitudes of actually detected defective portions are different although the defective portions have the same dimensions. Thus, the defect detection sensitivity of the inspection apparatus is expressed ambiguously.
FIG. 21 shows transmission light output characteristics obtained when a mask formed of a half-tone film (and glass) was observed with light of a proper wavelength. FIG. 41 show transmission light output characteristics obtained when a mask formed of a combination of a chrome film and a half-tone film was observed with light of a proper wavelength Specifically, overshoot and undershoot occur at edge portions of the glass/half-tone film, or a large difference occurs between a chrome level and a half-tone level.